Add a config object

- add the following file:
  + src/urban_meal_delivery/_config.py
- a config module is created holding two sets of configurations:
  + production => against the real database
  + testing => against a database with test data
- the module is "protected" (i.e., underscore) and imported at the
  top level via a proxy-like object `config` that detects in which of
  the two environments the package is being run

src/urban_meal_delivery/__init__.py

@@ -6,8 +6,11 @@ Example:
     True
 """
 
+import os as _os
 from importlib import metadata as _metadata
 
+from urban_meal_delivery import _config  # noqa:WPS450
+
 
 try:
     _pkg_info = _metadata.metadata(__name__)
@@ -21,3 +24,7 @@ else:
     __author__ = _pkg_info['author']
     __pkg_name__ = _pkg_info['name']
     __version__ = _pkg_info['version']
+
+
+# Little Hack: "Overwrites" the config module so that the environment is already set.
+config = _config.get_config('testing' if _os.getenv('TESTING') else 'production')

src/urban_meal_delivery/_config.py

@@ -0,0 +1,87 @@
+"""Provide package-wide configuration.
+
+This module is "protected" so that it is only used
+via the `config` proxy at the package's top level.
+
+That already loads the correct configuration
+depending on the current environment.
+"""
+
+import datetime
+import os
+import warnings
+
+import dotenv
+
+
+dotenv.load_dotenv()
+
+
+class Config:
+    """Configuration that applies in all situations."""
+
+    # pylint:disable=too-few-public-methods
+
+    CUTOFF_DAY = datetime.datetime(2017, 2, 1)
+
+    # If a scheduled pre-order is made within this
+    # time horizon, we treat it as an ad-hoc order.
+    QUASI_AD_HOC_LIMIT = datetime.timedelta(minutes=45)
+
+    DATABASE_URI = os.getenv('DATABASE_URI')
+
+    # The PostgreSQL schema that holds the tables with the original data.
+    ORIGINAL_SCHEMA = os.getenv('ORIGINAL_SCHEMA') or 'public'
+
+    # The PostgreSQL schema that holds the tables with the cleaned data.
+    CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA') or 'clean'
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<configuration>'
+
+
+class ProductionConfig(Config):
+    """Configuration for the real dataset."""
+
+    # pylint:disable=too-few-public-methods
+
+    TESTING = False
+
+
+class TestingConfig(Config):
+    """Configuration for the test suite."""
+
+    # pylint:disable=too-few-public-methods
+
+    TESTING = True
+
+    DATABASE_URI = os.getenv('DATABASE_URI_TESTING') or Config.DATABASE_URI
+    CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA_TESTING') or Config.CLEAN_SCHEMA
+
+
+def get_config(env: str = 'production') -> Config:
+    """Get the configuration for the package.
+
+    Args:
+        env: either 'production' or 'testing'; defaults to the first
+
+    Returns:
+        config: a namespace with all configurations
+
+    Raises:
+        ValueError: if `env` is not as specified
+    """  # noqa:DAR203
+    config: Config
+    if env.strip().lower() == 'production':
+        config = ProductionConfig()
+    elif env.strip().lower() == 'testing':
+        config = TestingConfig()
+    else:
+        raise ValueError("Must be either 'production' or 'testing'")
+
+    # Without a PostgreSQL database the package cannot work.
+    if config.DATABASE_URI is None:
+        warnings.warn('Bad configurartion: no DATABASE_URI set in the environment')
+
+    return config